1. Field of the Invention
This invention relates to a rotary shaft seal and, more particularly, to an annularly continuous, but axially variable seal for providing hydrodynamic lubrication between the seal and a rotating shaft. A system using differential pressures to keep the seal compressed as it wears is disclosed.
2. Description of the Background
This invention may be adapted for use with devices having tubular members which are supported for relative rotation by means of bearings contained within a lubricant chamber in an annular space between the members.
Such a tool is connected as part of a drilling string, and drilling fluid is circulated downwardly through a passageway formed by the tubular members and upwardly within the annulus between the tool and well bore. Thus, one such member often comprises a housing or case suspended from the lower end of the string, while the other comprises a hollow shaft rotatably supported by the housing and suspending a bit on its lower end. A motor, such as a turbine, is often disposed within the annular space between the housing and shaft for rotating the shaft and thus the bit without the necessity for transmitting torque from the surface to the housing through thousands of feet of drilling string. The passageway thus includes the annular space in which the turbine is disposed and which is connected by ports through the shaft to the bore of the shaft leading to the bit at its lower end. Typically in such tools, the housing is connected at its upper end to the drill string, and the shaft is caused to rotate with respect to the housing by means of the motor therebetween, whereby spiral blades or the like carried about the tubular shaft cause a reduction of pressure of drilling fluids as it passes through the blades and turns the blades. As in the case of a turbodrill, the motor often comprises turbine blades on the shaft and tubular members within the annular space between them. The drilling fluid which is circulated through the motor is confined for passage out through the bit.
Due to the drop in pressure across the turbine section and the bit, there is a substantial pressure differential across the seal means of drilling tools of this type. It is well known that a rotary seal means separating pressure in this manner is subject to considerably greater wear than one merely separating fluids at the same pressure. The problem of wear is even more serious in this environment due to abrasive particles in the drilling fluid which find their way between the surfaces of the seal means and the engaged tubular members. Another factor in seal wear is the severe axial and radial vibrations encountered in drilling boreholes through earth formations.
Unless protected therefrom, the bearings mounted in the annular space are also damaged by the abrasive particles in the drilling fluid. Since bearing replacement requires the raising and lowering of the drill string at great expense, efforts have been made to isolate the bearings from the abrasive drilling fluid by containing them in a lubricant chamber formed at least in part by a pair of seal means in the annular space. However, if one or both of these seal means also functions as a pressure separation element, it rapidly becomes worn and permits the abrasive drilling fluids to enter the lubricant chamber.
Because of the detrimental environment encountered by rotating motor parts positioned in a borehole drilling operation, many of the research efforts concerning the development of downhole "mud" motors for use in downhole drilling operations have been concerned with the protection of bearing and sealing moving surfaces from the abrasive effect of drilling fluids commonly referred to as drilling "mud." The following United States patents are particularly concerned with this problem: U.S. Pat. Nos. 3,659,662; 3,666,333; 3,971,450; 4,019,591; 4,222,445; 4,256,189; and 4,260,032. These patents deal with the problem of protecting bearing surfaces or the like from the drilling mud by a variety of schemes.
U.S. Pat. No. 3,659,662 discloses a tool which is intended to overcome this problem by providing a system in which a labyrinth is provided within the annular space between the shaft and housing. The shaft is ported in such a manner that the pressure drop is taken across the labyrinth and the pressure is equalized across the seal means forming the lubricant chamber in which the bearings are contained. However, if the passages through the labyrinth are small, they tend to be become clogged by the particles in the drilling fluid, thereby decreasing the cooling effect; and, if they are sufficiently large to pass a large volume of drilling fluid, the drilling efficiency of the bit is lowered considerably. Furthermore, even if the labyrinth is to be replaced by a third seal means of a non-leaking type, as disclosed in U.S. Pat. No. 3,971,450, it might still be subject to damage by the tendency of particles in the drilling fluid to enter between the sealing faces.
U.S. Pat. No. 3,666,333 incorporates a number of rather complex features in an attempt to solve this problem. This patent shows upper and lower seal systems located above and below the bearings and comprised of grease and oil filled chambers to prevent the entry of borehole fluids into the bearings. In addition, a labyrinth of mating annular parts are positioned above the upper sealing system to further impede the flow of drilling fluids, under the higher pressure of the drill pipe bore, from entering the rotating bearings and surfaces of the drilling motor.
Many devices such as that shown in U.S. Pat. No. 3,971,450 utilize seals having irregular configurations which, when unconfined in their assembly in the well tool, are subject to extrusion and deformation due to the heat and pressures developed in the borehole. These seals deteriorate and become ineffective to protect the moving parts from the borehole environment.
U.S. Pat. No. 4,222,445 uses a buffer fluid to protect a lubricant chamber. However, failing to take into account other important problems such as seal wear, the ability of this sealing system to protect the bearings is short lived.
Many approaches to this problem have simply attempted to use brute force, i.e., using a large seal area under high compression such as is shown in the lower seal of U.S. Pat. No. 4,019,591. This approach provides an initial seal but because of high friction and ensuing heat scorching of the seal, the seal surface wears and begins to leak lubricant.
U.S. Pat. Nos. 3,449,021; 2,867,462; and 3,831,954 deal with slanted sealing surfaces between tubular members. U.S. Pat. No. 3,449,021 illustrates such a seal for use between relatively rotating surfaces. However, the sealing contact surface against the rotating element is along a rather narrow band which in the configuration illustrated is not suited for adaptation to the environment of a borehole drilling apparatus. In addition, the harsh and inaccessible environment of the borehole renders it important to provide an effectual sealing surface that can be compensated for wear to avoid the necessity of early and frequent replacement. The above patents directed to slanted sealing surfaces all utilize irregularly shaped or nonconfined sealing members which would be deformed under the conditions in which the present invention is practiced. Further, the seals are not compensated for wear to provide a relatively long sealing life.
The characteristics of lubrication of hydrodynamic seals have been studied by the inventor and reported in several articles. Kalsi, M. S. and G. A. Fazekas, "Feasibility Study of a Slanted `O-ring` as a High Pressure Rotary Seal, " ASME Paper No. 72-WA/DE-14 (1972); Kalsi, M. S., "Elastohydrodynamic Lubrication of Offset O-Ring Rotary Seal," ASME Paper No. 80-C2/Lub-7 (1980) and in a dissertation of the same title submitted to the University of Houston in 1975.
The art has long sought a new and improved rotary shaft seal which will provide a simple but effectual seal in an environment of abrasive fluids while overcoming the difficulties and problems outlined above. The present invention overcomes these difficulties and provides a desirable sealing system by incorporating a continuous, axially variable hydrodynamic sealing surface between the shaft and housing of a downhole motor. Such a sealing system operates efficiently with a pressure differential across the seal and at a high temperature. Further, such a system provides a means for compensating for wear on the seal so as to maintain a seal of high integrity.